I was recently sent this photo with a request for advice on what I thought it was.
The masses of fluffy white fungal growth looks like white mold (sclerotinia sclerotiorum), a plant fungus that normally occurs because the bulbs have become damp during their dormancy. This can occur if not enough air movement is available when hung, or because they become too wet in the ground. It is also common in hyacinths and other bulbs.
While we have fungicides for commercial use, I am not aware of one available to gardeners.
The best thing to do is
1. Keep bulbs dry during dormancy. If they are dug, keep them in onion bags in a cool dry location. Many people hang them in small sheds and these can get very hot and at times very humid, so be careful to identify the suitable location. If left in the ground, make sure they are in well drained soil so when they do get wet during their dormancy they can quickly dry out.
2. If white mold is identified, get rid of infected bulbs (burn or bin) immediately.
Hope this is helpful.
All the blooming best, Charles